76 HUMAN PHYSIOLOGY. 



currents undergo a diminution of negative variation, a change which passes 

 along the nerve in the form of a wave and with a velocity equal to the rate 

 of transmission of the nerve impulse. The wave-length of a single negative 

 variation has been estimated to be eighteen millimeters, the period of its 

 duration being from 0.0005 to 0.0008 of a second. 



It is asserted by Hermann that perfectly fresh, uninjured muscles and 

 nerves are devoid of currents, and that the currents observed are the result 

 of molecular death at the point of section, this point becoming negative to 

 the equatorial point. He applies the term "action currents" to the currents 

 obtained when a muscle is thrown into a state of activity. 



Electrotonus. The passage of a direct galvanic current through a portion 

 of a nerve excites in the paits beyond the electrodes a condition of electric 

 tension, or electrotonus, during which the excitability of the nerve is decreased 

 near the anode or positive pole, and increased near the cathode or negative 

 pole; the increase of excitability in the catelectr atonic area that nearest 

 the muscle being manifested by a more marked contraction of the muscle 

 than the normal when the nerve is irritated in this region. The passage of an 

 inverse galvanic current excites the same condition of electrotonus; the 

 diminution of excitability near the anode, the anelectrotonic that now nearest 

 the muscle being manifested by a less marked contraction than the normal 

 when the nerve is stimulated in this region. Similar conditions exist within 

 the electrodes. Between the electrodes is a neutral point, where the cate- 

 lectrotonic area merges into the anelectrotonic area. If the current be a 

 strong one, the neutral point approaches the cathode; if weak, it approaches 

 the anode. 



When a nerve impulse passes along a nerve, the only appreciable effect 

 is a change in its electric condition, there being no change in its temperature, 

 chemic composition, or physical condition. The natural nerve-currents, 

 which are always present in a living nerve as a result of its nutritive activity, 

 in great part disappear during the passage of an impulse, undergoing a 

 negative variation. 



Law of Contraction. If a. feeble galvanic current be applied to a recent 

 and excitable nerve, contraction is produced in the muscles only upon the 

 making of the circuit with both the direct and inverse currents. 



If the current be moderate in intensity, the contraction is produced in the 

 muscle, both upon the making and breaking of the circuit, with both the direct 

 and inverse currents. 



If the current be intense, contraction is produced only when the circuit 

 is made with the direct current, and only when it is broken with the inverse 

 current. 



